1. Field of the Invention
The present invention relates to a broadband wireless communication system, and more particularly to a system and a method for performing handover in a Worldwide interoperability for Microwave Access (WiMAX) mobile communication system supporting a Point-to-MultiPoint (PMP) scheme.
2. Description of the Related Art
The next generation communication system is continuing to develop in such a way that a Mobile Station (MS) is offered various and plentiful high-speed services.
An exemplary instance of the next generation communication system is a WiMAX communication system corresponding to a communication system based on Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard.
In general, the WiMAX mobile communication system is based on IEEE 802.16e Wireless Metropolitan Area Network (WMAN) standards ensuring mobility of the MS, and supports Broadband Wireless Access (BWA). A network structure supported by the IEEE 802.16 standard is operates in accordance with two schemes, including a mesh scheme and the PMP scheme.
Presently, in some nations and regions, the WiMAX mobile communication system having the PMP structure is being used as a test or for commercial purposes. This WiMAX mobile communication system is worthy of close attention with respect to the aspects of high-speed data communications, the maximum communication range, and relatively cheap costs. However, problems have been raised in that the WiMAX mobile communication system having the PMP structure spends high costs in installing hot zones each of which functions as a wireless Local Area Network (LAN) base station (BS) for relaying radio waves so as to service a plurality of user MSs and their frequent movements, and installing the hot zones has no other option than to concentrate in a limited area, such as a crowded downtown, or a university library.
Therefore, a reform measure using Multiple Input Multiple Output (MIMO) or cell division technology is essential in respect to these hot zones.
In relation to this, FIG. 1 illustrates a general cellular system in which the prior cell is divided into multiple small cells. Usually, in a mobile communication system having the cellular structure, a base station (BS) controls one cell, and offers services to an MS located in the cell. Referring to FIG. 1, as a service coverage area is divided into multiple small zones through the cell division causing the radius of one cell to change from R to R/2, the same frequency is used in two cells that are far away from each other. A frequency reuse factor (i.e., the number of cells representing how many cells are assigned the total frequency band) can increase the total capacity of a cellular system in an environment like above. Therefore, a cell in which traffic congestion occurs is divided into smaller subcells or micro-cells, and there exists a BS in each subcell or micro-cell itself.
In this manner, since an increase of the number of cells is matched with the reproductively of cells, i.e., an increase in the reusability of cells, the cell division causes the total capacity of the cellular system to increase whereas a burden is imposed on control over an upper layer due to a frequent handover procedure between cells setting call from a base station (i.e., from a serving base station) of a current cell caused by the cell division to a base station of another cell target (i.e., to a target base station).